1. Field of the Invention
The present invention relates to circuits employing insulated gate bipolar transistors (IGBTs) and, more particularly, to power circuits using IGBTs as switches where the instantaneous junction temperatures of the IGBTs are of concern.
2. Related Art
IGBTs are often used in pulse width modulated (PWM) power circuits such as motor control circuits, power supplies, etc. The junction temperature of the IGBTs in such power circuits must not exceed critical temperature values during the operation of the circuits or else device and/or circuit failures will result. Designers of power circuits using IGBTs typically design the circuits with operating safety margins which arbitrarily limit the output power of the circuits in the hope of maintaining the junction temperature of the IGBTs well below the critical values.
Unfortunately, such operating safety margins may needlessly restrict the output power of the power circuits when conservative margins are used in the designs. It is therefore desirable to monitor the instantaneous junction temperature of an IGBT in a power circuit and limit the power output of the circuit only when the junction temperature of the IGBT exceeds a threshold. Such monitoring may be accomplished in closed loop fashion (employing temperature feedback) which would allow the IGBT to operate within its safe temperature limit without needlessly restricting output power.
FIG. 1 shows the relationship between voltage, current and junction temperature in an IGBT. The curves labeled "Voltage" represent the voltage across the collector and emitter terminals of an IGBT in a bridge circuit driving an inductive load when the IGBT is turned off. Naturally, at turn off, the current from collector to emitter in the IGBT ramps down after the voltage ramps up. As shown, the rate at which the IGBT voltage increases (dv/dt) is substantially higher at low junction temperatures than at high temperatures.